Substituent‐Controlled Energetics and Barriers of Mechanochromic Spiropyran‐Functionalized Poly(ɛ‐caprolactone)

Abstract

This work is dedicated to Prof. Brigitte Voit on the occasion of her 60 th birthday. In a joint theoretical and experimental study, it is shown that the onset of the mechanically-induced spiropyran (SP) to merocyanine (MC) isomerization can be controlled by both the regiochemistry and the substitution pattern of SP. Four SP-based bifunctional initiators with consistently varied polymer chain anchor point and substituent are used to synthesize poly-𝝐-caprolactone (PCL). Theoretical calculations (1S and 3S COGEF methods) and in-situ visible light absorption measurements of films during uniaxial stress-strain experiments consistently show varying activation barriers of the force-induced ring-opening reaction of SP to give MC. SPs with PCL chains attached in ortho-position to the pyran oxygen isomerize at lower stress than their para-analogs. NO 2 -substituted SP mechanophores exhibit a lower activation barrier compared with H-substituted ones, but only if the -NO 2 substituent is located in para-position relative to the O at the pyran half. These results are consistent with theoretical loading rate-dependent rupture forces required to break the C-O bond of SP and may guide mechanophore design.

Conformer Ring Flip Enhances Mechanochromic Performance of ansa-Donor–Acceptor–Donor Mechanochromic Torsional Springs

“…This parameter is important, as was shown for various systems. 23,47−49 Notably, in contrast to two-state systems where different regioisomers and positions of mechanophore linkage may influence barrier heights of the mechanochemical reaction, 47,49,50 in DAD springs a decrease of the D−A dihedral angle does not pass through any transition state, and hence a strong correlation of linkage mode with a mechanochromic response is possible.…”

“…Having identified dithienyl-flanked quinoxaline 4 as the most promising DAD system where torsion leads to a significant variation in optical and electronic properties, we turned our attention to different modes of covalently linking the spring to the polymer matrix. This parameter is important, as was shown for various systems. ,− Notably, in contrast to two-state systems where different regioisomers and positions of mechanophore linkage may influence barrier heights of the mechanochemical reaction, ,, in DAD springs a decrease of the D–A dihedral angle does not pass through any transition state, and hence a strong correlation of linkage mode with a mechanochromic response is possible.…”

“…The average transition barrier for the thiophene flip was estimated from 43 CoGEF simulations to be 0.39 eV for du -, uu -, and ud -conformers (Figure c). Despite the unreliability in the prediction of reactions and barriers with CoGEF, ,, the value obtained here can be regarded as the upper limit of the real flipping barrier. The characteristic time corresponding to this barrier is 570 ns at room temperature; that is, flipping is extremely fast and we may regard the isomers to be thermally distributed at each moment on the experimental timescale.…”

Mechanochemical Activation of Anthracene [4+4] Cycloadducts

“…For these values, the expected bond opening forces for the [4+4] C–C bonds are between 1.7 and 2.5 nN. These are rather large bond opening forces as compared to those of spiropyranes, ,,, ferrocenes, or cyclobutenes, , but in the range of measured forces for some cyclopropane ring openings . Due to thermal activation, the C–O bond breaking in the ester side group would happen only at considerably larger forces between 3 and 4 nN at room temperature and in the mentioned range of loading rates compared to >5 nN at zero temperature.…”

“…This led the authors to conclude that “these results may suggest that typical CoGEF calculations do not properly reflect the mode of mechanical activation for anthracene dimer mechanophores”. Similarly, CoGEF simulations of related Diels–Alder adducts with anthracene show a mixed picture in which the expected retro-Diels–Alder reaction happens sometimes and sometimes other, unexpected bonds break. , Related problems appear also in the CoGEF description of the mechanochemical activation of the spiropyran-merocyanine transition …”